MPLS (Multi Protocol Label Switching)

Multiprotocol Label Switching (MPLS) is a mechanism in high-performance telecommunications networks that directs data from one network node to the next based on short path labels rather than long network addresses, avoiding complex lookups in a routing table. The labels identify virtual links (paths) between distant nodes rather than endpoints. MPLS can encapsulate packets of various network protocols. MPLS supports a range of access technologies, including T1/E1, ATM, Frame Relay, and DSL.

MPLS Label
00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31
Label EXP: Experimental (QoS and ECN) S: Bottom-of-Stack TTL: Time-to-Live

MPLS uses a 32-bit label field that contains this information

– 20 bit label

– 3 bit experimental field (Cisco QOS)

– 1 bit bottom of stack indicator

– 8 bit TTL field

Labels can also correspond to other Parameters

  • Layer 3 VPN
  • Layer 2 circuit
  • Outgoing interface on egress router
  • QoS
  • Source Address

MPLS was designed to support forwarding of non-IP protocols as well


  • LSR(Label Switch Router ro Provide(P) Router
  • Edge LSR (E-LSR) or Provider Edge(PE) Router
  • Ingress E-LSR (Enpose)
  • Egress E-LSR (Dispose)
  • Customer Edge (CE) Router
  • Customer (C) Router


MPLS Database: FIB   and LFIB



Cisco Routers can be configured to disable MPLS TTL Propagation

MPLS Control Plane

* Control plane protocol used for MPLS Label information


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